The invention relates to a wire electrode and to a process for producing a wire electrode, especially for the spark erosion process, with a single-layer or multi-layer core, the outer layer of which consists of copper or a copper/zinc alloy having a predominant alpha phase fraction, and with a sheath layer consisting of a zinc or a zinc alloy.
Wire electrodes for the spark erosion process are produced, as a rule, with a high-strength core which, moreover, should also be a good electric conductor. Cores made from brass or composite cores, in which a steel nucleus is surrounded by a copper or brass layer, have proved appropriate for producing high strength. To increase the cutting capacity of wire electrodes of this type, a sheath layer, which consists, as a rule, of zinc or a zinc alloy, is coated onto these. Very good cutting capacities have been achieved with wire electrodes provided with a pure zinc coating. However, the cutting capacity of these electrodes decreases when tall workpieces are to be cut. The reason for this is that the pure zinc of the sheath layer evaporates rapidly and is therefore consumed in a very short time, so that the wire electrode then cuts with its core material, thus again reducing the cutting capacity as a whole.
Tests have shown that, where tall workpieces are concerned, better cutting capacities are achieved when the zinc of the sheath layer is a constituent of an alloy. It has proved advantageous, here, to produce the sheath layer from a homogeneous beta brass. An electrode of this type has a very good cutting capacity even in the case of tall workpieces. A disadvantage, however, is that such an electrode is relatively cost-intensive to produce. In this case, on the one hand, an accurate alloy composition of the core must be maintained and, on the other hand, diffusion has to be carried out over a long period of time at high temperature in order to achieve a state of equilibrium. This state subsequently has to be fixed by rapid cooling. It is extremely difficult here, in this known process, to control the phase fractions in the sheath layer accurately. Slight deviations in the production process lead to the presence also of alpha and/or gamma brass in the sheath layer in addition to the beta brass.
On the one hand, the object on which the invention is based is to design a wire electrode of the type initially mentioned, in such a way that it has an even better cutting capacity than a wire electrode with a sheath layer consisting of a pure beta phase, and, on the other hand, the object on which the invention is based is to propose a process for producing wire electrodes, by which process sheath layers consisting essentially of homogeneous gamma or epsilon brass can be cost-effectively produced.
This object is achieved, as regards the process, by means of the features of the present invention. The wire electrode is defined by the features shown hereinafter. The outer sheath layer consists of a gamma or epsilon phase. Advantageously, inert hard phases can also be intercalated into the gamma or epsilon phase, thus affording the advantage that the erosion and discharge behavior of the eroding wire for the eroded materials is further improved. These inert hard phases can be formed from diamond, boronitride, a conductive ceramic or graphite.
The choice of a pure gamma phase, to which, where appropriate, hard inert materials are added, has proved even more advantageous than the beta phase in terms of cutting behavior. With conventional processes, that is to say long-time diffusion, it is scarcely possible to produce such a gamma phase in pure form. As a rule, a mixed structure having fractions of the alpha, beta and/or gamma phase is obtained.
The invention proposes a process for producing homogeneous sheath layers from gamma or epsilon brass, which utilizes states of non-equilibrium as a result of extremely short diffusion times which are caused by a high heating and a high cooling speed and by a comparatively short holding time. Surprisingly, epsilon brass first forms, then gamma brass, the gamma brass in the sheath layer having a considerably higher growth rate than beta brass which, insofar as it forms at all in the short diffusion times proposed by the invention, then experiences merely a fraction of the growth of the gamma phase. The beta phases remain, in practice, below the detection limit and are revealed merely as very small margins which delimit, relative to the core region, the gamma phase extending over the entire sheath layer. By means of this process according to the invention, it is thus possible to produce pure epsilon or gamma phases, even in wire electrodes which, in the case of conventional diffusion annealing, would exhibit a juxtaposition of alpha, beta and gamma phases.
Core: CuZn5; galvanizing 30 xcexcm at 1.0 mm;
Drawing from 1.0 to 0.40 mm;
Annealing: heating at 200 K/s,
Annealing temperature=600xc2x0 C.
Cooling speed=300 K/sec;
Drawing from 0.40 to 0.25 mm
Core: CuZn5; galvanizing 30 xcexcm at 1.2 mm;
Drawing from 1.2 to 0.60 mm;
Annealing: heating at 40 K/s,
Annealing temperature=800xc2x0 C.
Cooling speed=60 K/sec;
Drawing from 0.60 to 0.25 mm